Rugged, narrow bandwidth mechanical filter

ABSTRACT

A mechanical filter includes a first plurality of active discs in spaced alignment, a second plurality of off-resonant discs with each disc of said second plurality positioned intermediate a pair of discs of said first plurality, coupling rods selectively attached to said discs, housing means and support means physically contacting said second plurality of discs and supporting said filter within said housing means.

United States Patent Peterson et a1.

[54] RUGGED, NARROW BANDWIDTH MECHANICAL FILTER [72] Inventors: Wesley D. Peterson, Santa Ana; Donovan A. Southworth, Costa Mesa, both of Calif.

[73] Assignee: Collins Radio Company, Dallas,

Tex.

[22] Filed: May 26, 1971 21 Appl. No.: 147,105

[ 51 Oct. 17, 1972 2,810,889 10/1957 Lundgren ..333/7l 2,930,006 3/1960 Hathaway ..333/7l FOREIGN PATENTS OR APPLICATIONS 810,523 3/1959 Great Britain ..333/71 Primary Examiner-J-lerman Karl Saalbach Assistant Examiner-Saxfield Chatmon, Jr.

Attorney-Henry K. Woodward and Robert J. Crawford [57] ABSTRACT A mechanical filter includes a first plurality of active discs in spaced alignment, a second plurality of offresonant discs with each disc of said second plurality positioned intermediate a pair of discs of said first plurality, coupling rods selectively attached to said discs,

housing means and support means physically contacting said second plurality of discs and Supporting said filter within said housing means.

3 Claims, 9 Drawing Figures PATENTEnum 11 I972 SHEEI 1 BF 2 FIG. I PRIOR ART FIG. 2

I'll-Inn'- FIG. 3

INVENTORS WESLEY D. PETERSON DONOVAN A. SOUTHWORTH BY' ,a/Ka/W ATTORNEY Sam 2 or 2 v 4 i sci:

FREQUENCY FlG.4c

FlG.4d

FlG.4e

' FIG. 4f

mozdlbdwm V FREQUENCY uwsmrons WESLEY o. PETERSON oouoww A. soumwonrn,

BY Zi/ ATTORNEY RUGGED, NARROW BANDWIDTH MECHANICAL FILTER This invention relates generally to electromechanical filters, and more particularly to mechanical filters of the stacked disc configuration.

Disc type mechanical filters are well known in the art. Typically, such filters comprise a plurality of stacked and spaced acoustically resonant discs with coupling wires selectively bonded to the discs to provide acoustical coupling therebetween. Since the discs cannot be supported without affecting their acoustical properties, the coupling wires must also provide the structural integrity of the structure, with support means provided only at the extremities of the coupling wires.

Such filters are especially applicable in radio communication equipment because of their relatively high Q, compared with electrical filter circuits. The center frequency, bandwidth, and filter skirt selectivity of mechanical filters are functions of disc size and number, spacing, and coupling wire dimensions. Importantly, bandwidth varies approximately as the total cross-sectional area of the coupling wires.

Practical limitations are encountered as filter bandwidth is narrowed, by reducing size of the coupling wires, because of structural strength considerations, As above stated, the coupling wires not only facilitate acoustical coupling between discs, but also provide structural integrity.

One technique for providing narrower bandwidths in such mechanical filters is described in U. S. Pat. No. 2,930,006 to Hathaway, assigned to the present assignee. In accordance with the Hathaway teachings, off-resonant discs, tuned to frequencies outside the bandpass of the filter, are placed intermediate the active discs which are tuned within the filter bandpass. The off-resonant discs, as will be shown later, reduce the coupling between the bandpass sections. Thus, larger diameter coupling wires may be employed for a given bandwidth than would be required without the presence of the off-resonant discs. However, this structure also has bandwidth limitations due to the struc tural considerations.

An object of the present invention is an improved, narrow bandwidth mechanical filter.

Another object of the invention is a narrow bandwidth mechanical filter which is especially rugged.

Features of the invention include a first plurality of active discs tuned to frequencies within the filter passband and arranged in spaced alignment, a second plurality of discs tuned to frequencies outside of said filter passband, each disc of said second plurality being intermediate and spaced from a pair of discs of said first plurality, a plurality of coupling wires selectively attached to said first and second pluralities of discs, and support means physically contacting and supporting at least one disc of said second plurality of discs.

These and other objects and features of the invention will be more readily apparent from the following detailed description and appended claims when taken with the drawings, in which:

FIG. 1 is a plan view of a conventional electromechanical filter removed from its housing;

FIG. 2 is a plan view of an electromechanical filter in accordance with one embodiment of the invention;

FIG. 3 is an end view of an electromechanical filter disc and further illustrating the support means of FIG. 2; and

FIGS. 4a-4f schematically illustrate the characteristics of a mechanical filter.

Referring now to the drawings, FIG. 1 is a plan view of a conventional mechanical filter shown generally a 10 designed for housing in a tubular case 12. In this embodiment, the mechanical filter comprises five active discs 14 which are supported in spaced alignment by means of coupling wires or rods 16 which acoustically couple the active discs. The extremities of coupling wires 16 are bonded to inactive: support members 18 and 20 at either end of the active discs.

Bonded in support member 18 is a coil assembly 22 which houses a magnetic coil and transducer rod 24 which extends through support member 18 and is affixed to the first active disc 14. This coil assembly and transducer rod may provide the filter input, and a similar arrangement is provided at the other end of the filter assembly for providing the filter output. Such an electromechanical filter drive arrangement is taught in U. S. Pat. No. 2,693,580 to Peterson, assigned to the present assignee.

To facilitate support of the filter assembly inside of tubular housing 12, grommets 26 and 28 mate with coil assembly 22 and provide the support of the filter as sembly with the inner walls of housing 12. A similar arrangement utilizing a base plate and standoff members is shown in U. S. Pat. No. 2,693,580, supra. In either support arrangement, the active discs 14 are not and must not be physically contacted other than by the coupling rods. Any other contact would detrimentally affect the acoustical qualitities of the discs and thus restrict or destroy the effectiveness of the filter.

As above described, the bandwidth of such an electromechanical filter, dependent on the total cross-section of the coupling rods, is limited because of the necessary structural integrity provided by the coupling rods. In accordance with the present invention, off resonant discs are placed intermediate pairs of active discs, in accordance with the teachings of Hathaway, supra, and support means is provided which physically contacts one or more of the off resonant discs. Applicant has discovered that such contact with the off resonant discs does not substantially affect the characteristics of the filter, and such support means obviates much of the need for the physical integrity of the filter attributable to the coupling rods.

A modification of the filter illustrated in FIG. 1, in accordance with the present invention, is illustrated in FIG. 2. In this embodiment, off resonant discs 40 are provided intermediate each pair of active discs 14, and support sleeve 42 construzted of molded plastic, metal, or the like is provided with dimples 44 (illustrated in the partial cutaway view) arranged to contact the end support member 18 and each of the off resonant discs 40. Support sleeve 42 is designed] to wrap around the disc assembly with the dimples engaging each disc at approximately spacing.

FIG. 3 isan end view illustrating an off resonant disc 40 with the attached coupling rods 16 and sleeve 42 arranged in supporting position. The: dimples 44 of sleeve 42 contact disc 40 at approximately 90 spacing and between coupling rods. Sleeve 42 along with grommets 28 and 26 thus provide support within tubular housing 12 for the filter assembly.

In alternative embodiments, the off resonant disc support means may comprise individual grommets for each off resonant disc, and in still another embodiment, the tubular housing may be indented after assembly to provide the support for the off resonant discs. While the number of contacting dimples obviously must be a minimum of three, this number is not critical. Further, while it is desirable that the dimples be equally spaced about the off resonant discs, this spacing is not essential.

The effect of employing off-resonant discs according to the Hathaway teachings will be further appreciated by considering FIGS. 4a-4f. FIG. 4a shows an analog circuit representing three discs as the shunt tanks 50, connected together with coupling wires represented as the series coupling inductors 52. FIG. 4b shows a plot of reactance versus frequency for any one of the shunt tank circuits 50, with F, representing the tuned frequency or resonance condition. If the center tank circuit of FIG. 4a were tuned to a frequency well above the filter passband, it can be seen that in the filter passband this tank circuit would appear simply as a shunt inductor. The circuit then is a tee of 3 inductors coupling the remaining two tank circuits as shown in FIG. 4c. Using textbook equations, this tee of inductors can be transformed into a pi of inductors as shown in FIG. 4d. The shunt inductors in the pi can then be absorbed into the two remaining tank circuits as shown in FIG. 4e, with only a slight change in the frequency of the tank circuits.

The significant change from FIG. 4a to FIG. 4e is that the remaining series coupling inductor has been transformed to be a much larger inductance than the original. Thus to realize the same filter bandwidth, one must increase the cross-sectional area of the coupling wires, which in turn results in a more rugged physical structure.

FIG. 4f shows as a solid line the reactance of a shunt tank circuit or disc versus frequency, and shows as a dotted line the reactance versus frequency for the same disc which has been gripped or physically supported in accordance with the present invention. It can be seen that there are very significant effects near resonance, notably a large reduction in Q," and a shift in the resonant frequency. However, if this disc is tuned so that the frequency f, shown in FIG. 4f is in the filter passband, very little change occurs in the value of inductive reactance at that frequency, and this does not have any.

substantial effect on the characteristics of the filter.

Narrow bandwidth filters have been constructed in accordance with the present invention which have not only narrower bandwidths, but are also much stronger and rugged. The filters can successfully withstand physical shocks which are destructive to conventional mechanical filters.

While the invention has been described with reference to specific embodiments, the description is illustrative and is not to be construed as limiting the invention. Various modifications and changes may occur to those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.

fA i ghanical filter comprising a first plurality of active discs tuned to frequencies within said filter passband and arranged in spaced alignment, a second plurality of discs tuned to frequencies outside of said filter passband, each disc of said second plurality being inter mediate and spaced from a pair of discs of said first plurality, a plurality of coupling wires selectively attached to said first and second pluralities of discs, housing means, and support means physically contacting and supporting said discs of said second plurality in said housing means, said support means comprising a sleeve member having raised portions at selective points to contact said discs.

2. A mechanical filter in accordance with claim 1 wherein said housing means comprises a cylindrical member.

3. A mechanical filter comprising a first plurality of active discs tuned to frequencies within said filter passband and arranged in spaced alignment, a second plurality of discs tuned to frequencies outside of said filter passband, each disc of said second plurality being intermediate and spaced from a pair of discs of said first plurality, a plurality of coupling wires selectively attached to said first and second pluralities of discs, cylindrical housing means, and support means physically contacting and supporting at least one disc of said second plurality of discs in said housing means, said support means comprising indented portions of said cylindrical housing means. 

1. A mechanical filter comprising a first plurality of active discs tuned to frequencies within said filter passband and arranged in spaced alignment, a second plurality of discs tuned to frequencies outside of said filter passband, each disc of said second plurality being intermediate and spaced from a pair of discs of said first plurality, a plurality of coupling wires selectively attached to said first and second pluralities of discs, housing means, and support means physically contacting and supporting said discs of said second plurality in said housing means, said support means comprising a sleeve member having raised portions at selective points to contact said discs.
 2. A mechanical filter in accordance with claim 1 wherein said housing means comprises a cylindrical member.
 3. A mechanical filter comprising a first pluralIty of active discs tuned to frequencies within said filter passband and arranged in spaced alignment, a second plurality of discs tuned to frequencies outside of said filter passband, each disc of said second plurality being intermediate and spaced from a pair of discs of said first plurality, a plurality of coupling wires selectively attached to said first and second pluralities of discs, cylindrical housing means, and support means physically contacting and supporting at least one disc of said second plurality of discs in said housing means, said support means comprising indented portions of said cylindrical housing means. 